Highly discriminative filter and bias-level gating circuit



B. E. cowAR'r ETAL 2,848,713 HIGHLY DISCRIMINATIVE FILTER AND BIAS-LEVEL GATING CIRCUIT 3 Sheets-Sheet 1 Filed Feb. 3, 1955 .ummmw wauw/Many .N SNN Aug. 19, 1958 B. E. cowART ETAL 2,848,713

HIGHLY DISCRIMINATIVE FILTER AND BIAS-LEVEL GAIING CIRCUIT Filed Feb. 3, 1955 3 Sheets-Sheet 2 f Jaa Aug. 19, 1958 B. E. cowAR'r ETAL 2,848,713 HIGHLY nIscRIMINATIvE FILTER AND BIAS-LEVEL GATING CIRCUIT Filed Feb. 5, 1955 3 Sheets-Sheet 3 nited States HIGHLY DISCRllVIINATIVE FILTER AND BIAS-LEVEL GATING CIRCUIT Application February3, 1955, Serial No. 485,974

6 Claims. (Cl. 343-8) t This invention relates to a highly discriminative filter and bias-level gating circuit and, more particularly, to a filter and gating circuit where Doppler frequency signals, indicating the presence of a moving object, may be highly accentuated over noise or other clutter.

While the invention may have a multitude of applications, it is particularly useful in velocity tracking systems where moving targets are detected as Doppler frequency signals. The invention allows target detection in such a system with a high degree of discrimination against unwanted component frequencies resulting from rain, noise, or other clutter and consequently aids in increasing the range of target acquisition. A system of this general type is found in copending U. S. patent applica- .tion Serial No. 492,627 for Velocity Tracking System for j'Increasing the Range of Acquisition of Moving Targets .'by Lloyd David Ball et al. filed March 7, 1955.

According to the basic principle of the invention, the texpected Doppler frequency spectrum is separated into .a plurality of band pass intervals, each interval covering ;a frequency range of Af/n, where Af represents the fre- .quency region covering the Doppler frequency spectrum for the range of expected target velocities and n is the number of band pass intervals employed.

Any signals obtained through the separate frequency intervals are individually detected and compared with a threshold bias-level corresponding to the expected statistical noise level in that particular frequency interval. The bias level detection technique employed is alternative or independently operative in the sense that any one of the separately filtered signal and noise components may be compared as an individual component with a noise-proportioned bias level to produce a moving-targetindicating output signal. Thus, in this manner, statistically White noise, considered to be at a constant llevel throughout the frequency spectrum Af may be effectively divided into n discrete frequency intervals so that the effect of noise in a single interval is l/n times its eect prior to spectrum subdivision.

While theV spectrum division parameter n may vary considerably, the theoretical maximum is a function of the expected frequency spectrum for a moving target considering the scanning rate and target size. In other words, the scanned target Doppler spectrum width, the center :frequency of which is referred to hereafter as fd, varies as `llteni; C)

since any greater number n would result in a substantial decrease in Doppler signal power. It is assumed here that the noise bandwidth of each of the filters preceding the detectors is C. P. S. and that this bandwidth does not appreciably reduce the signal power compared to a very wide or non-discriminating filter.

Another specific contribution of the invention lies in a circuit technique of moderate predetection filtering, detection, and then additional postdetection filtering to further emphasize the low frequency components resulting from detection. This technique, it will be shown, provides a considerable improvement in signal-to-noise ratio without requiring the complexity of the system described above.

The contribution of the invention may be demonstrated as an improvement over the optimum detectability or minimum threshold signal 4conditions typical of the practice of the prior art. For this purpose the conventional approach to the signal separation problem discussed above will be considered Where Af is 2150 C. P. S. and TW is 8 milliseconds. The band pass region Af then corresponds to the intermediate frequency band B specified on page 220 of the M. I. T. Radiation Laboratory Series, volume 24, entitled Threshold Signals. The parameter TW corresponds to the reciprocal of the factor r utilized in this book to represent the duration of the signal. Thus factor BT for the assumed situation becomes 2150 times 1/125 or 17.

In referring to the signal threshold curves shown on page 220 of this book, it will be noted that the optimum design according to the prior art technique is approximately +9.5 db, where the video bandwidth b is selected so that the factor br is equal to .4.

If, according to the present invention, the frequency division parameter n is selected so that optimum predetection filtering is achieved, the factor B1- becomes equal to l, since effectively the bandwidth B is equal to the reciprocal of the useful signal duration Tw. In this case then, referring again to the curves on page 220 of Threshold Signals, the optimum predetection filter design allows a relative minimum signal threshold level of -l- 4 db where the factor br is in the range lObl. This means that it is possible to decrease the minimum signal threshold level by 5.5 db from the level required according to conventional practice and still maintain the same probability of detecting useful signals. This improvement may be slightly offset due to noise accumulation resulting from mixing through the bias-level gate; the loss being l in the order of .5 to 1 db. The resultant improvement, however, is still considerable, being in the order of 4.5 to 5 db.

An improvement of lesser degree may be accomplished with a simpler circuit by utilizing the combination technique of predetection and postdetection filtering, according to the other technique of the invention. In this situation assume that the parameter n is reduced to 5 so that the factor Br is approximately 3.3. Then reference again to the curves of page 220 in Threshold Signals shows that the relative minimum signal threshold is in the order of +6 db for a video bandwidth factor b1=.4. In this case then the relative improvement provided by the invention is 3.5 db.

In its basic structural form the invention comprises a plurality of filtering means for separating applied Doppler frequency signals variable throughout a region Af into .y n separate intervals, and a plurality of detectors and biastion filtering technique is realized, each ofthe plurality,

of lteringmeans in the basic embodiment'ofthe invenl. tion includes a predetection filter, a detector fcircuit, and a postdetection filter which may be a simple RC low pass filter.

Although a multitude of specific types of filtersV mayv be utilized in practicing the invention, two basic filter types of predetection filters are illustrated, one based on the well-known m-derived principley and the other being more specifically designed for the purposes of the present invention in order to economize on circuit arrangements.

In a similar manner, although many types of detectors, postdet'ection filters, and bias level gating circuits are known in the art, two specific arrangements are shown which are suitable for utilization in the present invention in order to aid those skilled in the art in practicing the invention. According to one of the species shown, a positive output signal is pro-duced indicating the detection of a Doppler signal above an expected noise level in a corresponding frequency division, and in the other a negative output' signal is produced indicating the same situation.

Another improvement resulting from the employment of the bias-level gating circuits of the present invention is that only the Doppler signal having the largest amplitude is detected. The reason for this is that the common biasing output "c'ircuit to which the noise-level signal is applied raises the bias level as soon as the Doppler signal is detected. This means that only` the Doppler signal having a level greater than any other'Doppler signal present will eventually pass as an output signal. Thus, in this manner, the invention provides a circuit which vnot only discriminates against noise but alsoy against weaker Doppler signals.

Accordingly, it is 'an object of the present invention to provide a highly discriminative Doppler frequency filter and bias level gating circuit which may be utilized to detect Doppler frequency signals indicating the presence of a moving object where the Doppler frequency signals are highly accentuated over noise or other clutter.

Another object is to provide a circuit for discriminating moving-object-indicating signal frequencies over noise frequencies with large improvement in signal-to-noi'se ratio.

A more Vspecific object is to provide a Doppler frequency filter and bias .level gating circuit wherein the expected frequency region Af is separated-into n divisions increasing the signal-to-noise ratio. f

Another specific object is to provide an efficient Doppler frequency filter and bias level gating circuit wherein predetection and postdetection filtering are utilized to-achieve a high signal-to-noise ratio without requiring a considerable number of complex circuits. n

An additional objectisto-provide a circuit for detecting frequency components representing a moving object where the circuit may be utilized in a velocity tracking system, the circuit allowing a target detection in such a system with a high degree of discrimination against-signal frequencies resulting fromrain, noise, or other clutter. v

The novel features which are believed to bec'haracteristic of the invention, both as to itsorganizatin and method of operation, together with further objects and advantages thereof, will be better understood, from "the following description considered in connection lwifh"'the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Fig. 1 is a block diagram of the basic embodiment of Doppler frequency filter and bias level gating circuit according to the present invention;

Fig. la illustrates a typical transmitted radar pulse;

Fig. lb illustrates a typical Doppler frequency spectrum resulting from the refie'c'tion-"of the pulse of Fig. la from atypical moving target; n

Fig. 2 is a schematic diagram of one form of predetection filter circuit set suitable for utilization in the embodime'nt of Fig. l;

Fig. 2a illustrates the attenuation characteristics of the various divisions'in the circuit of Fig. 2;

Fig. 3 is a schematic diagram of another form of predetect'ion Afilter set which may be utilized in the embodiment of Fig. 1; v

Fig. 3a illustrates the attenuation characteristics of the various filter divisions, in the circuit of Fig. 3;

, Fig. 4 isa schematic-diagram ofpa suitable set of detectors, postdetection filters, ,andlbias level gating circuits which may be utilized Yin the embodiment of Fig. l; and

Fig.` Sis a schematic diagram of another suitable set of detectors, postdetection filters, and bias level gating circuits which may be utilized in the embodiment of Fig. l. l Referencefisr'now` made tovFig. l wherein the basic embodiment'of Doppler rfrequency filter and bias-level gating cicu'it is 'shown in block diagram form. As shown in Fig.v l-, the basic embodiment comprises a first means 100 fr se'pai'ati'rg applied Doppler frequency signals into a plurality Yof 4separate frequency intervals, where n` intervals are specified, .n being any integer. A particular' form of means 100 is sho-wn but will 'not be presently describedsincethe invention is "not 'so limited. The output 'signals produced by means 100 'are applied to a second means 200 `operative as a bias-level gating circuit t'o detect any Doppler Afrequency 'signal produced by means V100 and to compare "suchva detected signal with a predetermined bias-level set in proportion to average noise amplitude f over an expected Doppler v frequency region Af.

vMeans 200 provides A'an output signal indicating the presence of a Doppler -frequencysignah in any frequency division 'of 'rrieans 100, having a level greater than the bias level Vpr'c'aportional -to average noise. Essentially, means 200`is alternative-in its operation since a signal produced vby'any one ofthe/frequency divisions in means may be'detected rand 'compared with the bias level proportional noise. vIn this .manner the output signal produced by circuit 200 effectively-"may be considered to represent a comparison in a single frequency interval of thgoutputs ofi-means 100. v

The basic principles of the invention may be better understood by considering the frequencyspectrum resulting from scanning a point target as shown in Fig. lb. In this figure an amplitude spectrum is shown rather than a power spectrum. As shown in Fig. lb, the amplitude spectrum is symmetrical about vthe Doppler frequency component fd. This value of this Vfrequency component (fd) may be derived through well-known formulas, such as those shown on page`288 of a book entitled Radar Engineering by 'Finkpublished' in 1947 by'The lMcGraw- Hill Bookv Company, New York and London. As shown on page 288 of this article, fthe Dopplerfrequency or Doppler shift Af '(not the "Af of the present invention) is defined asfollows:

where Af -corresponds tojd ofthe present specification, fr is vthe frequency received Vafter transmission, .fo is the transmitted frequency, v is the -velocity of the target, and cis thevelocity of light. v

In terms of atypical application, fd in cycles per second ('C. P. 'S.) maybe defined by' the equation: fd=v 3l-l` where `v is in terms'ofiknots. If a'target velocity is VK19 K Edition, first printing, 1956.

knots, for example, then the Doppler frequency which resin kt 2 as indicated in Fig. la.

lAs indicated in Fig. 1b, the amplitude spectrum 'has a value of at the frequency points fd-Tw and fd-l-Tw. As a result most of the useful Doppler frequency power may be obtained in the region which isv l/2Tw C. P. S. to each side of the Doppler frequency fd. The minimum bandwidth of the frequency intervals provided by means 100 therefore should be no less than the frequency width l/Tw. Consequently, in the typical example pointed Aout 'above where Tw=8 milliseconds, the minimum frequency bandwith of the intervals in means 100 becomes 125.0 C.P. S.

Although the invention is not limited to particular filter types, it may prove helpful to those skilled in the art in practicing the invention to illustrate two suitable types of filters which may be employed. Reference is made to Fig. 2, therefore, illustrating a suitable type of filter including five sections providing ve frequency divisions as follows:

C. P. S.

Section 100-1 600-1030 Section 100-2 1030-1460 Section 100-3 1460-1890 Section 100-4 1890-2320 Section U-5 2320-2750 This division covers an expected frequency range of 600-2750 C. P. S.

While the values obtained for the embodiment of the invention shown in Fig. 2'are not for predetection filters of the well-known m-derived type, these filters may be so designed. The design formulas for the well-known mderived type filters are provided in chapter 6 of a book entitled Reference Data for Radio Engineers by Federal Telephone & Radio Corporation published by American Book-Stratford Press, New York, New York, 4th

One specific type of mderived filter which may be employed is shown on page 178 and referred to as a six element series band pass ilter.- The design formulas for this circuit are found on page 179.

`The attenuation characteristics of the filter divisions of Fig. 2 are shown in Fig. 2a where i't will be noted that the attenuation function is inverted with respect to that shown on page 178 of the book entitled Reference Data for Radio Engineers. It will also be noted that the actual band pass characteristic does not have infinite attenuation and a sharp cut-off at the specified frequency divisions due to the fact that the inductances are not lossless.

It will be noted that the circuit elements in each division have identifying symbols with a subscript corresponding to the `division number. Thus all of the elements in division 100-1 have the subscript 1. It will also be noted that where elements may have the same circuit value the same identifying symbol is utilized. Thus capacitors C11 in circuit 100-1 have the same value. It will also be noted that the identifying superscript and symbol remain the same throughout the various divisions where the corresponding elements occupy the same position in the respective circuits. Suitable circuit element values for providing the desired frequency division as specified above may then be provided as follows:

Capacitor C11 microfarads-- .0434 v Inductor L11 millihenrys-- 946 Capacitor C12 microfarads .065 Inductor L12 millihenrys-- 630 Capacitor C13 microfarads-- .089 Inductor L13 rnillihenrys 462 Capacitor C14 microfarads-- .296 Inductor L11 millihenrys-- 138 Filter section 100--2 Capacitor C51 microfarads .0215 Inductor L31 millihenrys 785 Capacitor C32 microfarads-- .0322 Inductor L22 millihenrys-.. 525 Capacitor C33 microfarads-- .0735 Inductor L33 millihenrys-- 229 Capacitor C24 y microfarads .245 Inductor L34 millihenrys.. 68.8

Filter section 100-3 Capacitor C31 microfarads-- .011 Inductor L31 millihenrys-.. 836 Capacitor C32 microfarads .0164 Inductor L32 millihenrys-- 558 Capacitor C33 microfarads-- .0785 Inductor L33 millihenrys 117 Capacitor C31 microfarads .261 Inductor L34 millihenrys 35.2

' Filter section 100-4 Capacitor C41 ....microfarads .00871 Inductor L41 millihenrys 663 Capacitor C12 rnicrofarads..- .0131 Inductor L42 millihenrys-- 442 Capacitor C42 microfarads .062 Inductor L13 millihenrys 93 Capacitor C44 microfarads .207 Inductor L54 millihenrys-- 27.9

Filter section 100-5 Capacitor C51 microfarads .00726 Inductor L51 millihenrys.. 548 Capacitor C52 microfarads .0109 Inductor L52 millihenrys-- 365 Capacitor C53 microfarads .0513 Inductor L53 millihenrys..- 77.4 Capacitor C54 microfarads .171 Inductor L54 mil1ihenrys 23.2

Filter section -1 An alternate form of filter which may be utilized to provide the desired frequency operation is shown in Fig. 3. The filter of Fig. 3 does not have as sharp a cut-oil characteristic as that of Fig. 2, as is illustrated in the attenuation characteristics shown in Fig. 3a, but is desirable due to the simplicity of circuits as well as due to the almost complete lack of ringing as compared to lters having very steep skirts.

Referring now to Fig. 3, it is noted that only three llter sections are shown, represented as 100-1, 10U- 8, and 100-16, indicating that there are a total of 16 similar sections, only three being shown for convenience.

The sixteen sections provide frequency divisions,

measured at the -1.5 db points, as follows:

' C. P. S. Section 100-1 293- 447 Section 100-2 447- 601 Section 100-3 601- 755 Section 100-4 755- 909 Section 100--5 909-1063 Section 100-6 1063-1217 Section 100-7 1217-1371k Section 100-8 1371-1525 Section 100-9 1525-1679f Suitablecircuit' values for .these sections. may then` be specified as follows:

Filter section .10d-1 CapacitorY C1 Inductor Li Capacitor-C Inductor L12..

Filter Capacitor C21 Inductor Lz! Inductor La Filter Capacitor C31 Inductor L31-. Capacitor C37.-- Inductor La Filter CapacitorCiL Inductor L41 Inductor L42...

Fi l ter Capacitor C51 Inductor L51-. Capacitor C. Inductor L52 Filter'V Capacitor Cal Inductor Lal.. Capacitor Cn?. Inductor La Filter Capacitor C51. Inductor Lg1 Capacitor C12. Inductor L5 Filter Capacitor Cal. Inductor Lal.- Capacitor Cn.

Inductor La F lter Inductor LN2 *.25 microfarads niet... i QlL-sloo section 100-2 .25 microfarads 345mi1lihenrys }QL 414 12800 micromicrofarads. 7.86 henrys section 100-3 pil-8182' .25 microfarads 209.7 millihenrys 8500 micromicrofarads 6.84 henrys section 100-4 .25 microfarads 141.2 millihenrys- 5990 micromtcrofarad 6.34 henrys sectionk I 00-5 .25 microiarads v Y.

6.27 henrys iQl-m 4section 100-6 76.6 millihenrys iQl'i'sgO 6.17 henrys I iQLU- section 100-7 .25 microfarads- 6.10 henrys :iQLlg'g section 100-8 .25 microfarads 47 .7 millihenr'y..

2020 micromicrofurads. 6.05 henrys section 100-9 .2.5 microfarads gemeente- .s ich m crom cro ara 5.98 henrys }Q0-24.5

section 100-10 .25 mlcrofarads.

5.92 henrys kiwi-26.8

Filter section 100-11 Capacitor C111 Inductor Lul.-

.25 mlcrofarads- 27.9 millihenrys- 1157 micromicrofarad 5.94 henrys Filter section 100-12 Capacitor Cn* Inductor Lul. Capacitor C112 IILdllCOI LN7.-

.25 microiarads 23L9 millihcnrys.- 992 micromicrofarads 5.95 henrys Filter section 100-13' Capacitor Cl.. In uctor Lul- Capacitor C13 Inductor Lisi.

Inductor L141.

Inductor LN2- Filter Section o-14 5.97 henrys Filter section -1-5 Capacitor C151 .2"5! microfarads;

the filter sections off the type described in Figs. 2 and 3 may be utilized directly in'bias-level gates 200, suitable circuits being shown in Figs. 4 and 5; it may be desirable to achieve further frequency discrimination by means of the detection` and postdetection filtering technique pro vided by the invention. It will be understood, however, in describing Figs. 4 and 5` below, that the bias-level gates 200 need not be utilized in connection with detectors 100, andpostdetection lilters 102, where the additional dis crimination is not desired.

Referring now to Fig. 4, it is noted that in one specic form detector may comprise resistors R111 and diodes D112. In Fig. 4 it will be noted that the anodes of diodes D112 are connected to corresponding resistors R111, whereas in the embodiment of Fig, 5 the cathodes of diodes D112 are coupled to corresponding resistors R111. The reason for this variation is that positive signals are detected in the embodiment of Fig. 4, and negative signals are rdetected in the embodiment of Fig. 5.

Signals thus detected are applied to postdetection filter sections 120, which may include an RC lter section 121 including a resistor R121 and a capacitor C121. The junction of resistor R121 and capacitor C121 is connected to the grid of'a'n'ampliier T122 having its output load resistor R122 connected to the grid of a cathode-follower circuit T124. The output signal of cathode-follower cir# cuit T124 contains the low frequency components of the input signal provided by detector 110.

It will be noted that the postdetection lter in the embodiment ofFig. 5 may be identical to those utilized in the embodiment of .Fig 4.

Suitableforms of. bias-level gating circuits for passing positive'a'nd negative signals are shown in Figs. 4 and 5, respectively. The bias-'level gating circuit 200 shown in Fig. 4 includes a series of diodes D201-1, D201- 2, D201--n, having anodes connected to the cathode-follower circuits T124-1, T124-2, T125-n, respectively, where the symbol n is utilized to indicate that the number of diodes D201 is varied according to the number of lter sections utilized.

Each of the diodes' D201 has an input load resistor R201, having one end connected to the anode of the corresponding diode and the other end connected to ground. In addition, there is associated with each diode D201, a second diode D202 having its cathode connected to the junction of resistors R201 and diode D201 and its anode connected to ground. Diodes D202 are operative to prev vent negative signals from passing through diodes D201, sincev such signals find a low load impedance path to ground.

The cathode of each diode D201 is connected to a biaslevel signal generating network 210 producing a signal representing the expected statistical noise level through the region Af. As shown in Figs. 4 and 5, circuit 210 may include a potentiometer including a resistor R212, the center tap of resistor R212 being connected to an output resistor R213. Suitable positive potential is applied to a resistor R211, resistor R211 being in turn connected to'resistor R212.

From the foregoing description it is apparent that the present invention provides a highly discriminative filter and bias level gating circuit Where Doppler frequency signals present in a frequency region are -separated into a plurality'of frequency divisions or intervals and then coin- `pared Ito noise :levelrepresenting signals in the respective.

. 9 t intervals. The technique of the invention makes it.pos sible to increase the accentuation of Doppler frequency signals, which may indicate the presence of a moving ob ject over noise or other clutter.

It should` also benoted that any signal passing through one of the diodes D201 also serves to add to the bias level provided by circuit 210. Thus, in particular, a signal is developed across resistors R212 and R213 in addition to thenoise-,level signal applied to resistor R11. This means,

then, that only the largest signal passing through one of the input filters is effective over the combined bias level of the noise-level signal and the other Doppler signals which may be present.

While the invention has been shown in a particular form where signals detected in any frequency interval are compared to a single bias level signal representing statistically averaged noise throughout the region, it will be understood that the basic concept taught herein is applicable as well to a utilization where the noise level represented -techniques by optimizing pre and postdetection ltering in the conventional manner. It has been shown that an improvement in the order of 5 db is possible where the optimum predetection filtering technique of the invention is employed and that an improvement of the order of 3 db is possible where the combination technique of predetection and postdetection filtering is utilized.

While only a few modifications have been shown by way of specific circuits herein, it will be apparent to those skilled in the art that a wide variety of circuits fall within the generic class provided by the invention.

It will be noted that as defined herein the term noise is intended to include clutter or other unwanted signals. Consequently, in the appended claims the following terms will be used interchangeably: noise; unwanted signals; unwanted frequency components; and clutter.

What is claimed is:

l. A highly discriminative circuit for detecting Doppler frequency signals present in a frequency region Af, the region Af including noise signals statistically averageable throughout the region, said circuit comprising: a plurality of filtering means responsive to received Doppler frequency signals for separating the signals throughout the region Af into a corresponding plurality of separate frequency divisions; a plurality of bias-level gating circuits coupled to said filtering means, respectively, each of said gating circuits including a unidirectional device having an input electrode for receiving the signal produced by the associated filter and an output electrode for passing a signal of a predetermined polarity; means coupling said output electrodes together; means providing a common bias circuit for said second electrode; and means for applying an average noise-level-representing signal to said last-named means, whereby said gating circuits may pass only a Doppler frequency signal having a level greater than the noise level represented, the largest Doppler signal received creating a further bias discriminating against any other Doppler signal of lesser amplitude.

2'. A circuit for detecting moving-target-indicating signals, where the frequency of the signals represents the velocity of the target, the variation of velocity expected and corresponding frequency variation being representable as the frequency difference Af, the moving-targetindicating signals being mixed with noise signals existing in the frequency difference region Af, said circuit comprising: n frequency separating circuits for receiving any signals in the frequency difference-region' Afand producing corresponding output signals in one of n separate frequency divisions; n gating circuits coupled to said n frequency separating circuitsjrespectively, each of said gating circuits including a diode having input and output circuits arranged to pass signals of a predetermined polarity; a common impedance coupled to the output circuits of said diodes; and means for applying a bias signal to said comomn impedance having a level selected to prevent unwanted frequency components in the region Af from passing through the corresponding gating circuits along with a moving-target-indicating signal in the particular frequency division, said common impedance also serving to discriminate against all moving-target-indicating signals except that having the largest amplitude.

3. In a system for velocity tracking moving targets, a circuit for detecting the presence of a moving target by the presence of corresponding Doppler signals, the Doppler signals existing in a frequency range corresponding to the range of velocity variation ofthe expected targets, the system being subject to unwanted signals in particular frequency intervals representable as a bias signal having a corresponding level, said circuit comprising: a plurality of first means for receiving the Doppler signals, each of said first means being operable to pass a different portion of the frequency range, where the bandwidth of the portion is at least as large as the expected bandwidth for the Doppler signals representing a moving target, said first means including a predetection filter for passing Doppler signals in the corresponding portion of the frequency range, a circuit for detecting the signals produced by the associated predetection filter, and a post-detection filter for receiving the detected signals and for passing the low frequency component thereof; and a plurality of second means coupled to said first means, respectively, each of said second means lbeing operable to combine a received Doppler signal passed through the corresponding first means with the corresponding unwanted signal representing bias signal and to produce an output signal representing the part of any signal received having a level greater than said bias signal.

4. A circuit for accentuating vDoppler frequency signals in a frequency region over noise statistically averaged throughout the region, the noise being representable as a gating control signal, said circuit comprising: a plurality of filters for receiving the Doppler frequency signals, each ofv said filters having a bandwidth equal to said region divided by the number in said plurality and covering a different portion of said region, each of said filters including a first circuit for pretetection filtering to produce a first output signal corresponding to a Doppler signal in the associated frequency portion, a second circuit for detecting said first output signal to produce a second output signal referenced to zero frequency, and a third circuit responsive to said second output signal to produce a third output signal containing the low and zero frequency components of said second output signal; and a corresponding plurality of gating circuits coupled to said filters, respectively, and responsive to the gating control signal for passing a received Doppler signal in the corresponding portion of the frequency region which is greater than the statistically average noise throughout said region.

5. The circuit defined in claim 4 wherein each of said second circuits includes a resistor and a diode, said diode having an anode connected to said resistor and having a cathode for receiving a reference potential, said second circuitsy detecting positive signal portions of said first output signal; and wherein each of said gating circuits includes a first diode and a load resistor, said first diode having an anode connected to said load resistor and a cathode for receiving said gating control signal, each gating circuit further including a second diode having a cathode connected to the junction of said first diode and said resiston and havingan an0de, ,for receiving said reference potential.

6. ,The circuit, defined-in claina'pet wherein each of said' secondsA circuitsrincludes, a resistonanda diode, said diode having a cathode connected to said resistor and having an .anode fo'rl receiving a reference potential, said second circuits detecting negative signal portions of said rst output signal;,and wherein each of said gating circuits'in-`v cludes a load resistor and a first diode having an anodey for' r receiving said gating 'control signal, each gating circuit fur-i 10 tial.

References Cited inthev4 ilef'of this. patent' UNITED STATES PATENTS f Llewellyn July 31, i19341fn. Dallos: Jan., 6,'41942.; 

